Silver is known for its antimicrobial properties, particularly when incorporated into or onto medical devices. However, it is challenging to coat or incorporate silver onto a surface, whether medical devices or other surfaces (e.g., seeds, plants, metals, etc.). Many products formed using existing silver compounds are poorly soluble, exhibit poor silver release profiles, the silver is inactivated in body fluids, and the antimicrobial activity is designed for planktonic bacteria and shows little or no effect against biofilm. Further, many, if not all, higher oxidation state silver compounds (e.g., oxysilver nitrate and silver (Ill) periodates) cannot readily be mixed with commercial gel formulations; typically the silver compound does not mix well, or the silver compound reacts with the gel, resulting in loss of its antimicrobial activity and/or loss of gel properties.
One conventional approach for obtaining antimicrobial medical devices is the deposition of metallic silver directly onto the surface of the substrate (for example, by vapor coating, sputter coating, or ion beam coating). However, these noncontact deposition coating techniques suffer many drawbacks, including poor adhesion, lack of coating uniformity, and the need for special processing conditions, such as preparation in darkness due to the light sensitivity of some silver salts. One particular drawback of these coatings is that the processes by which the coatings are formed do not adequately coat hidden or enclosed areas, such as the interior lumen of a catheter or stent. Additionally, these methods produce coatings that act like metallic silver in that they do not release silver from the coating and require contact with the coating to provide any antimicrobial action. Because they do not release sufficient silver ions into aqueous fluids, they offer little or no protection from bacteria carried into the body upon application of the device and do not inhibit infection in the surrounding tissue or fluid.
Another method of coating silver onto a substrate involves deposition or electrodeposition of silver from solution. Drawbacks of previous deposition methods include poor adhesion, low silver pick-up on the substrate, complex manufacturing processes, the need for surface preparation, high labor costs, and the need for additional deposition agents and stabilizing agents.
Some silver coatings release, to varying degrees, silver ions into the solution or tissue surrounding the substrate. However, activation of such coatings often requires conditions that are not suitable for use with some medical implants. These conditions include abrasion of the coating surface, heating to a temperature above 180° C., contact with hydrogen peroxide, and treatment with an electric current.
Alternatively, a solid form of the silver salt can be mixed with a finely divided or liquefied polymeric resin, which is then molded into the article. Further, the antimicrobial compound can be mixed with monomers of the material prior to polymerization.
There are several disadvantages to this last approach: larger quantities of the antimicrobial material are required to provide effective antimicrobial activity; it is also difficult to produce articles that actually release the antimicrobial ions because most coatings absorb little, if any, water to aid in the diffusion and release of the antimicrobial ions, resulting in articles that provide only a limited antimicrobial effect.
There is also a need for compositions that overcome the solubility, settling, and agglomeration problems of conventional antimicrobial compositions, and exhibit enhanced, sustained release of antimicrobial agents. There is also a need for antimicrobial compositions that may be incorporated into a gel or hydrogel used to make or coat a device, while retaining its antimicrobial effectiveness. There is also a need for antimicrobial compositions that are stable, e.g., thermally stable, light stable, stable in the materials they are included on/with, and are not inactivated in the environment of their intended use.